1. Field of the Invention
This invention relates to the use of SiC for electromagnetic radiation sensing and resistance control.
2. Description of the Related Art
Acoustic absorption, bandgap absorption and impurity absorption refer to three different radiation absorption mechanisms. Acoustic absorption results from increasing lattice vibration under radiation exposure. This is a desirable absorption mechanism when detection is sought over a broad range of wavelengths, since it causes the resistance of a SiC chip to increase approximately linearly with the irradiating energy over an appreciable wavelength range. SiC absorbs heat throughout its volume, not just along its surface, and therefore has a high thermal capacity for absorbing high laser doses and other applications. While it would be desirable to employ SiC for detecting infrared (IR) radiation and other lower wavelength radiation, acoustic absorption has not been observed in SiC for wavelengths below about 10 micrometers, thus eliminating part of the IR band along with shorter wavelengths.
Bandgap absorption occurs at a specific wavelength corresponding to a material's bandgap energy (the energy differential between its conductance and valence bands). SiC can occur in more than 70 different polytypes, each of which has its own distinguishing bandgap energy as measured parallel to the c-axis of a single crystal. The cubic crystalline form of SiC (referred to as 3C-SiC or β-SiC) has the lowest energy bandgap (approximately 2.3 eV), with the longest corresponding wavelength (approximately 0.55 microns) of all SiC crystal structures and polytypes, but this is yellow/green light entirely outside the IR range. The bandgaps of all other SiC crystal structures and polytypes are higher in energy with shorter wavelengths, making SiC unsuitable for bandgap absorption detection of IR.
With impurity absorption, a dopant is introduced into a host material and radiation is detected at the energy differential between the host and dopant conductive band energy levels (for an n-type dopant) or valence band energy levels (for a p-type dopant). The resistance of a chip of host material drops exponentially with the radiation energy at the specific wavelength corresponding to the impurity absorption energy but not at other wavelengths, resulting only in a discrete detectable wavelength. Impurity absorption has been observed with SiC in the IR range (Air Force Materials Laboratory, “Silicon Carbide Absorption”, Hughes Aircraft Company Electronic Properties Information Center, pages 9–16), but only at specific wavelengths corresponding to particular impurities. No IR absorption mechanism over a broad band has been discovered.